Alkaline batteries (particularly nickel-cadmium batteries) have assumed increasing importance in commercial markets both for home appliances and in many industrial applications. Desirable properties of alkaline batteries are high capacity per unit weight, high charge and discharge rates and long shelf life. Rechargeable home appliances are becoming more and more evident in the marketplace. Methods of manufacture have emphasized not only increased energy storage per unit weight but also increased charge and discharge rates. Aside from nickel-cadmium batteries, nickel electrodes are also useful in other alkaline batteries such as nickel-hydrogen batteries, nickel-zinc batteries and nickel-iron batteries.
A commercially established procedure for fabricating nickel electrodes for alkaline batteries is to impregnate a porous supporting electrode structure (i.e., a porous nickel plaque) with finely divided active material so as to present a high surface area of active material to the battery electrolyte. While loadings obtained in this fashion are quite satisfactory, higher loadings are desirable and reduced time for loading is economically advantageous. In addition, it is desirable to formulate procedures which result in more rapid and more efficient commercial production of these electrodes.
A number of impregnating procedures have been used in the past. Particularly simple is the procedure of soaking the porous plaque in a salt solution and evaporating the liquid. This step is followed by soaking the plaque in a second solution to convert the soluble salt to an insoluble active form. Soaking in either the first or second solution, or both, might be repeated several times to increase loading. Thermal decomposition is also used to convert the salt into an insoluble active form. These procedures are generally referred to as chemical impregnation processes.
An alternative approach over the soaking processes is electrolytic impregnation (see L. Kandler, U.S. Pat. No. 3,314,355 issued Oct. 26, 1965). In this process, active material is continuously deposited directly in the pores of the plaque. Here the impregnation is carried out in an acid electrolyte containing cations of the active material. In the electrolysis process, the nickel plaque is made the cathode, and the cations of the active material as well as reducible ions (for example, nitrate ions) migrate into the pores of the plaque. Only the reducible ions are reduced at the cathode (in the plaque) because of their more positive potential. During this electrolytic reduction, hydrogen ions are consumed making the region inside the plaque more basic. This results in precipitation of the cations in the form of active material. This method is a further improvement on previous methods and is more adaptable to mass production.
Loading levels are increased somewhat by repeated electrolytic impregnation and overnight drying between each impregnation. However, this process modification increases manufacturing time. More rapid impregnation is achieved by increasing the temperature of the electrolyte, as described in R. L. Beauchamp, U.S. Pat. Nos. 3,573,101 issued Mar. 30, 1971 and 3,653,967 issued Apr. 4, 1972. However, even more rapid impregnation than achieved up to the present time is highly desirable, especially where a continuous impregnation procedure is used in the commercial production of electrodes. Attempts to increase loading rates by increasing the electrolytic current result in the production of a hard crust of active material on the outer portion of the nickel plaque which does not contribute to the capacity of the battery and prevents impregnation in the pores of the nickel plaque. This results in low load levels.